Apparatus for determining a functional index for stenosis assessment

ABSTRACT

An apparatus for determining a functional index for stenosis assessment of a vessel is provided. The apparatus comprises an input interface (40) and a processing unit (50). The input interface is configured to obtain image data (30) representing a two-dimensional representation of a vessel (6). The processing unit (50) is configured to determine a course of the vessel (6) and a width (w1, w2) of the vessel along its course in the image data and is further configured to determine the functional index for stenosis assessment of the vessel based on the width of the vessel in the image data.

CROSS-REFERENCE TO PRIOR APPLICATIONS

This application is a continuation of U.S. application Ser. No.16/338,290, filed on 29 Mar. 2019, now U.S. Pat. No. 11,179,043, whichis the U.S. National Phase application under 35 U.S.C. § 371 ofInternational Application No. PCT/EP2017/075021, filed on 2 Oct. 2017,which claims the benefit of European Patent Application No. 16191781.0,filed on 30 Sep. 2016. These applications are hereby incorporated byreference herein.

FIELD OF THE INVENTION

The present invention relates to an apparatus for determining afunctional index for stenosis assessment of a vessel, a method fordetermining a functional index for stenosis assessment of a vessel, arelated computer program product and a computer-readable medium havingstored said computer program product.

BACKGROUND OF THE INVENTION

Cardiovascular diseases are a leading cause of death in theindustrialized world. The predominant form of cardiovascular diseaseresults from the chronic build-up of a fatty material (“plaque”) in theinner tissue layer of the arteries supplying the heart, brain, kidneysand lower extremities. Such build-up of plaque in the arteries may leadto a narrowing of the lumen of the vessel, which narrowing is referredto as a stenosis.

Progressive coronary artery disease such as vessel stenosis restrictsblood flow to the heart. Assessing a blood flow through a vessel mayrequire various examination procedures, be it invasive or non-invasiveones. Due to the lack of accurate information provided by currentnon-invasive tests, many patients require invasive catheter proceduresto assess a blood flow through a vessel.

Recent studies have demonstrated that hemodynamic characteristics, suchas fractional flow reserve (FFR), may be important indicators to assistin determining optimal treatment for a patient with arterial disease.Some of the conventional assessments of the fractional flow reserve useinvasive catheterization to directly measure blood flow characteristics,such as pressure and flow velocities. However, these invasivemeasurement techniques present a risk to the patient and may result insignificant costs.

FFR is an index of the functional severity of a stenosis that iscalculated from pressure measurements, preferably made duringarteriography. The FFR may be defined as the distal blood pressure(behind a stenosis or downstream of it when viewed in flow direction ofthe blood) relative to the proximal pressure (behind the stenosis orupstream of it when viewed in flow direction of the blood) underhyperemic conditions (i.e. the ratio between the pressure after a lesionand the normal pressure). In other words, the fractional flow reserveexpresses the maximum flow down a vessel, in particular in the presenceof a stenosis compared to the maximal flow in the hypothetical absenceof the stenosis. The fractional flow reserve is a normalized value inthe range between 0 and 1, wherein the fractional flow reserve of 0.5indicates that a given stenosis causes a 50% drop in blood pressure andthus restricts the maximum blood flow capacity in the vesselsignificantly.

Alternatively, the Instantaneous Free-Wave Ratio (iFR) may be used as anindicator for the remaining flow maximum flow capacity.

However, the amount of effort required for stenosis assessment may bevery high and may require a lot of information, in particular fornon-invasive stenosis assessment.

WO 2014/072861 A2 describes determination of a fractional flow reservebased on certain extracted features. The features are extracted from avolumetric representation of a region of interest of a patient's body.Furthermore, boundary conditions for determining an FFR via simulationare determined and these boundary conditions can be used to classify theunknown FFR.

SUMMARY OF THE INVENTION

In view of the above, there may be a need to reduce the effort requiredfor stenosis assessment and to simplify the process therefor.

This need is fulfilled by the subject-matter of the independent claims,wherein further embodiments are incorporated in the dependent claims. Itshould be noted that the following described aspects of the inventionapply also for the method, the computer program element and thecomputer-readable medium, at least in an analogous manner.

According to a first aspect of the invention, an apparatus fordetermining a functional index for stenosis assessment of a vessel isprovided. The apparatus comprises an input interface and a processingunit. The input interface is configured to obtain image datarepresenting a two-dimensional representation of a vessel. Theprocessing unit is configured to determine a course of the vessel and awidth of the vessel along its course in the image data and is furtherconfigured to determine the functional index for stenosis assessment ofthe vessel based on the width of the vessel in the image data.

The input interface may receive image data in any data format, whereinthe image data correspond to a two-dimensional representation of aregion of interest of a patient's body. The image data may be capturedby various possible image capturing devices as long as the image dataare a two-dimensional representation of a region of interest containingat least one vessel, in particular at least one blood vessel. In otherwords, the input interface is adapted to receive two-dimensional (2D)image data. The input interface may be an optical capturing unit whichis adapted to scan an image. The processing unit may be adapted toprocess the scanned image data and to identify a blood vessel as well asto determine the functional index for stenosis assessment based on thewidth of the blood vessel. The width may be, in particular, an internalwidth of the vessel such that the width identifies the space or diameteravailable for the blood flow. Alternatively, the input interface may beadapted to receive data representing a two-dimensional image, which dataare provided by an external unit.

The image data may be provided by an X-ray imaging system, wherein thevessel or vessels in a region of interest are projected onto atwo-dimensional image plane while the blood flowing through thevessel(s) contain a contrast agent such that the internal diameter ofthe vessel is represented in the image. Thus, the 2D image data isacquired using X-ray angiography. In particular, angiography images ofthe coronary arteries of the patient may be acquired.

The functional index for stenosis assessment may be, for example, afractional flow reserve indicator. However, other indicators, such asthe above mentioned iFR, may be possible, as well.

According to a further aspect of the invention, a method for determininga functional index for stenosis assessment is provided, wherein themethod comprises the following steps:

a) obtaining image data corresponding to two-dimensional image data of avessel;

b) determining a course of the vessel and a width of the vessel alongits course in the image data;

c) determining a functional index for stenosis assessment of the vesselbased on the width of the vessel in the image data.

For example, the two-dimensional image data may be a projection of aspatial object, e.g., a body having at least one vessel, onto atwo-dimensional surface resulting in a two-dimensional image of the atleast one vessel.

The approach described herein and the findings the approach is based onmay be summed up as follows:

Quantitative assessment of stenosis in the human arteries is highlydesirable for interventional decision making. Current concepts typicallyinclude purely geometric assessment of stenosis diameter, length,cross-sectional area and derived quantities. These can be determinedfrom a single 2D angiographic projection (2D QCA, quantitative coronaryangiography) or from multiple 2D projections or rotational sequences (3DQCA).

Recently, a shift from geometric stenosis assessment towards functionalassessment is happening. Functional measures also consider the impact ofthe stenosis on blood flow. Specifically, virtual Fractional FlowReserve (vFFR) aims to predict measured fractional flow reserve (FFR).Virtual FFR can be calculated by applying computational fluid dynamics(CFD) to a 3D vessel geometry model from CT image volumes or 3Dangiographic datasets.

For workflow simplicity, it would be highly desirable to generate afunctional index using a single 2D projection image.

This is done in accordance with the apparatus and the method describedabove substantially as follows:

-   -   determining a geometric model of at least a section of a vessel,        the model including an inner diameter and a change of the        diameter along the course of the vessel;    -   determining at least one resistance value of at least the        section of the vessel based on the geometric model;    -   determining a pressure drop of a fluid which flows through said        vessel or section of the vessel based on the at least one        resistance value.

The determined pressure drop is an equivalent of the functional indexfor stenosis assessment.

These and other aspects of the present invention will become apparentfrom and be elucidated with reference to the embodiments describedhereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention will be described in thefollowing with reference to the following drawings:

FIG. 1 schematically illustrates an imaging system.

FIG. 2 schematically illustrates two-dimensional image data for beingused by an apparatus according to an exemplary embodiment.

FIG. 3 schematically illustrates an apparatus according to an exemplaryembodiment.

FIG. 4 schematically illustrates a flow chart of a method according toan exemplary embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS

In the following, the invention is exemplarily described as being usedin the context of the apparatus for determining a functional index forstenosis assessment. However, the invention can also be used in thecontext of the method for determining a functional index for stenosisassessment. Thus, all the following examples and/or explanations mayalso be intended as being implemented by the method of the invention.Features relating to the apparatus are to be understood as to similarlyapply to the method, as well, and vice versa.

FIG. 1 schematically illustrates an imaging system 10, for example anX-ray system, for generating image data of a region of interest of anobject 5, for example of a part of the human body. The imaging system 10may emit X-ray radiation towards and through the object such that thecomposition of the object is projected onto the projection surface 20.Thus, a two-dimensional image is generated which can be used as inputdata for the apparatus described herein.

However, it should be noted that image data generated by other means orby other processes may be used as input data for the apparatus as longas the image data are a two-dimensional representation of a spatialobject like the human body.

FIG. 2 schematically shows an image 30 containing a projection of asection of a vessel 6 in a two-dimensional projection. In thistwo-dimensional image, the section of the vessel 6 does not have aconstant diameter but the diameter changes. In particular, the diameterW1 at the left side is greater than the diameter W2 at the right side ofthe image 30.

The functional index for stenosis assessment for the shown section ofvessel 6 is determined as follows:

-   -   determining a geometric model of at least a section of a vessel,        the model including an inner diameter of a vessel or a section        of the vessel and the change of the diameter along the course of        the vessel. This may be done, for example, by partitioning the        section of vessel 6 as to have multiple partitions and        determining a diameter or mean diameter for each partition; the        higher the numbers of partitions, the more exact may be the        functional index for stenosis assessment;    -   determine at least one resistance value of at least the section        of the vessel based on the geometric model. With reference to        FIG. 2 , a simulated fluid flowing from the left to the right        would experience an increasing fluid dynamic resistance if W2 is        smaller than W1, for example due to a stenosis. In particular,        the vessel or vessel segment may be approximated by at least one        linear or non-linear resistor, preferably a series of such        resistors.    -   determine a pressure drop of a fluid which flows through said        vessel or section of the vessel based on the at least one        resistance value. For example, the pressure drop may be        determined using a lumped element fluid dynamics model,        including the determined resistance value or values and certain        boundary conditions. For example, assumptions may be made        regarding the (volumetric) flow rate and/or pressure at the        inlet and/or outlet of the vessel or vessel section, which        assumptions are then used as boundary conditions in the lumped        elements model. Alternatively, physical measurement values, such        as an aortic pressure measurement, may be used as boundary        conditions.

The determined pressure drop, for example over a stenosis in a coronaryvessel segment, is an equivalent of the functional index for stenosisassessment.

FIG. 3 shows an apparatus for determining a functional index forstenosis assessment. The apparatus comprises an interface 40 forreceiving image data representing a two-dimensional image, a processingunit 50, and an output unit 60. Even though the output unit 60 is shownin FIG. 3 , this is not a necessarily required component of theapparatus. The processing unit may determine the functional index forstenosis assessment and may store this value in a memory unit or storageunit such that an external output unit may access said memory unit orstorage unit to read out the functional index for stenosis assessment.

The interface 40 is configured for receiving image data 30. The imagedata may be provided as pictures or projections of body regions or asdigital data transmitted to the interface 40 via a suitable datatransmission protocol either via a data transmission network or byaccessing kind of memory unit which stores the image data.

The output unit may be a device for optically indicating the functionalindex for stenosis assessment. For example, the output unit may be amonitor or any other kind of display.

According to an aspect of the invention, the input interface isconfigured to obtain image data representing a two-dimensionalrepresentation of a vessel and the processing unit is configured todetermine a course of the vessel and a width of the vessel along itscourse in the image data and is further configured to determine thefunctional index for stenosis assessment of the vessel based on thewidth of the vessel in the image data.

The input interface may receive image data in any format. The inputinterface is adapted to receive two-dimensional image data, inparticular. Especially, only a single two-dimensional representation (aprojection, for example) of a vessel is used for determining thefunctional index for stenosis assessment, for example a fractional flowreserve.

According to an embodiment, the two-dimensional image data of the vesselrepresents a two-dimensional image of a blood vessel of a human.

According to an embodiment, the two-dimensional image is a projection ofthe vessel.

According to an embodiment, the width of the vessel corresponds to adiameter of the vessel in an image plane or projection plane.

As an effect, coronary lesion hemodynamic significance may be estimatedbased on fractional flow reserve, for example.

According to an embodiment, the processing unit is configured to apply adensitometry method to the image data as to compensate forforeshortening effects in the image data when determining the width ofthe vessel.

Densitometry is used for quantitative measurement of optical density. Bymeans of the densitometry method, it may be determined if the distanceof the vessel from the image plane or projection plane varies along thecourse of the vessel and this may be additionally considered whendetermining the width of the vessel. This may be done as to notadulterate the determined width value of the vessel as a result of thevarying distance from the image plane or projection plane. In otherwords, the measured width may be corrected based on the results of thedensitometry method.

The densitometry may be used to compensate for foreshortening in theprojection/two-dimensional image and the width may be determined basedon both the diameter in the projection and the result of thedensitometry measurement by accumulating the results of theseapproaches.

According to an embodiment, the processing unit is configured to apply ascaling factor to the width of the vessel in the image and to determinethe functional index for stenosis assessment based on the widthmultiplied by the scaling factor.

Thus, a non-magnified size of the vessel may be determined and thefunctional index for stenosis assessment is determined based on thiscorrected vessel diameter.

According to an embodiment, the processing unit is configured to receivethe distance of the projected vessel from a projection plane of theprojected image data and to determine the scaling factor based on saiddistance.

The magnification of the vessel width in the image or projection isdependent on the distance of the vessel from the image or projectionsurface, respectively. This distance may be considered when determiningthe scaling factor to get a more accurate result.

According to an embodiment, the processing unit is configured to segmentthe vessel along its course such that there are multiple vessel segmentsor partitions and to apply a specific scaling factor to the width ofeach one of the multiple vessel segments or partitions.

As a projection of the vessel or a two-dimensional image of the vesselare used, the width along the course of the vessel in the projection orimage may not be true to scale. Therefore, it may be necessary to applydifferent scaling factors to the width of the segments of the vessel inorder to determine the non-magnified width.

According to an embodiment, the processing unit is configured to segmentthe vessel such that one segment of the vessel has substantially thesame distance from the projection surface.

Thus, there is an appropriate segmentation such that there is almost thesame magnification of the diameter within one segment and it may beensured that the scaling factor is applicable to the entire segmentwithout artificially adding distortion or bias to the width of thevessel.

The segmentation may be done such that the distance of the vessel withinone segment is within a corridor of a predetermined width around themedium distance of the segment, for example within 5% or 10% around themedium distance.

According to an embodiment, the processing unit is configured to detecta reference element within the image data and to determine the scalingfactor based on a known size of the reference element and the size ofthe reference element in the image data.

Thus, the scaling factor of the vessel may be determined based on theknown dimensions of the reference element in a more accurate manner.

According to an embodiment, the processing unit is configured todetermine a functional index for stenosis assessment based on the widthof the vessel in the image data, wherein the functional index is one of:a pressure drop along a centreline of the vessel, a virtual fractionalflow reserve, a curve of the pressure drop as a function of the bloodflow through the vessel, a fluid-dynamic resistance value, a bloodvelocity profile, a blood velocity distribution.

In general, any parameter or any functional index that can be derivedfrom hemodynamic parameters (e.g. coronary flow reserve, CFR, instantflow reserve, iFR) may be used individually or in combination with anyone or multiple of the abovementioned indicators.

According to an embodiment, the vessel is an artery of a human body.

FIG. 4 schematically illustrates a flow chart 100 of a method fordetermining a functional index for stenosis assessment. The methodcomprises the following steps:

In a first step 110, also referred to as step a), image datacorresponding to two-dimensional image data of a vessel are obtained.

In a second step 120, also referred to as step b), a course of thevessel and a width of the vessel along its course in the image data isdetermined.

In a third step 130, also referred to as step c), a functional index forstenosis assessment of the vessel is determined based on the width ofthe vessel in the image data. It is understood that, without repeatinghere all the explanations, examples, features and/or advantages providedwith reference to the apparatus for determining a functional index forstenosis assessment, the method of the invention is intended to beconfigured to carry out the method steps 110 to 130 for which theapparatus is configured to. Thus, all the above examples, explanations,features and/or advantages, although provided previously with referenceto the apparatus for determining a functional index for stenosisassessment, are also intended to apply in a similar manner to the methodand to, in particular for the following exemplary embodiments of themethod.

According to an exemplary embodiment of the method, the step ofobtaining image data comprises obtaining a projection of the vessel,wherein the width of the vessel is determined based on the projection.

According to an exemplary embodiment, the method further comprises thesteps of: determining a length of the vessel in the image; determining ascaling factor based on a densitometry approach; applying the scalingfactor to the determined width of the vessel.

According to a further example of the present invention, a computerprogram element is provided, which, when being executed by a processingunit is adapted to carry out the method described above.

According to further example of the present invention, a computerreadable medium having stored thereon a program element is provided,which, when being executed by a processing unit is adapted to carry outthe method described above.

In other words, the approach described above relating to the apparatusand method for determining a functional index for stenosis assessmentmay be summed up as follows and it is proposed here to approximate thisby one or more of the following:

Apply 2D QCA methods to obtain a 2D segmentation and a vessel centerlineof the culprit vessel segments. In order to estimate the non-magnifiedsize of the vessel diameter and length, the scaling factor due tomagnification can be estimated from either of the known system geometryand an estimate of the position of the heart inside the X-ray system,the known size of the catheter at the ostium of the contrary arterytree, or a phantom/reference element placed on the chest wall of thepatient.

Thus, a geometric model including a centerline and the local vesselradius r_(i) for each centerline point are determined. This may includebranching points.

The geometric model further includes cross-sectional areas that areestimated for one or more of the centerline points. For example, suchestimate is made for each centerline point. In a first approximation,this is A_(i)=π*r_(i) ².

Thus, a simple geometric model of the segmented vessel is generated,which is used as a basis for a subsequent calculation of a functionalindex.

In one embodiment, densitometric information is used to estimate thevessel diameter in the through-plane direction. This may be based oncardiac digital subtraction angiography (DSA). Furthermore, this mayutilize a time series images over a complete cardiac cycle to improvethe robustness of the densitometric measurement. Densitometricevaluation might be limited to a segment of the vessel, e.g. between twobifurcations.

In another embodiment, the projection angle is chosen such that theapparent stenosis diameter is minimized. This choice may be donemanually by a human operator of an imaging system by selecting one ofmultiple acquired projections, or an automatic suggestion is made basedon prior-knowledge using a reference database of projection images. Thisapproach may result in a systematic underestimation of thecross-sectional area, and may lead to improved reproducibility ascompared to an unguided approach.

In another embodiment, the projection angle is chosen (manually orautomated) such that least vessel foreshortening occurs. This may becombined with the diameter minimization.

Based on some or all of the vessel characteristics that are extractedfrom 2D images (like diameter, length, curvature, through planediameter, bifurcation number and location), a functional index forstenosis assessment is calculated. This functional index can, forinstance, be either of: the pressure drop along the centerline of thevessel, virtual FFR, e.g. based on CFD simulations, a curve of thepressure drop as a function of the blood flow through the stenosis, or aquantity derived thereof, a fluid-dynamic resistance value, simulatedaverage blood velocity profile or velocity distribution, or any otherfunctional index that can be derived from hemodynamic parameters (e.g.CFR, iFR, etc.).

In an embodiment, the functional index for stenosis assessment isderived from a 2D projection image. Preferably, a single angiographic 2Dprojection image with segmentation and pressure drop profile is used forfunctional assessment, as it can be obtained from fluid dynamicssimulations.

However, additionally, the index may be independently derived from aplurality of 2D images. That is, a functional index being derived“independently” involves deriving separate indices for individual imagesin a series, for instance carrying out the vessel segmentation, modelingand fluid dynamics simulation for each image of the series.

Different images in the series are for example acquired in differentcardiac states. Alternatively or in addition, different projectionangles may be used providing different viewing angles on thevasculature.

Optionally, an improved assessment can be based on one or more of thefunctional indices derived from different images. For example, in thiscase, it may be helpful to merge the individual numbers into a combinedindex (e.g. the mean value).

Alternatively or in addition, a variation between the functional indicesfrom different images is determined. For example, such variation can becompared to a predetermined maximal variation (Vmax), based on which anacceptance criterion may be generated, in order to have feedback on theaccuracy of the results. If the simulated values from multiple imagesare within expected or predetermined limits of variation, the acceptancecriterion may indicate that the simulated results may be accepted withhigher confidence than for a single frame evaluation.

The expected maximal variation (Vmax) may be predetermined taking intoaccount the nature of the different images of the sequence. For example,for multiple images taken at the same projection angle but in differentheart phases, Vmax may be lower than for multiple images taken atdifferent projection angles.

Alternatively or in addition, the processing unit may be configured tocalculate a quality score for one or more images being used as a basisfor calculating the functional index for stenosis assessment.

For example, such quality score may be based on one or more of thefollowing quality parameters:

-   -   The resolution of the angiographic X-ray image, as may for        example be determined from DICOM data associated with the image.    -   The size of the projected vessel tree, e.g. an image area        covered by contrast-enhanced vessels. A larger projected        vascular tree may represent a higher quality image.    -   The amount of noise in the projection, as may for example be        determined by means of a noise measurement in a background area,        i.e. a portion of the image outside the vascular tree. A lower        amount of noise may represent a higher quality image.    -   A sharpness of projection, as may for example be determined by        means of an entropy analysis or gradient measures.    -   The motion of the coronary arteries, as may for example be        determined by means of a correspondence analysis with        neighbouring images in a sequence. Strong motion may lead to        blurred contours and thereby a lower quality image.

Other quality parameters, such as the amount of foreshortening in theprojection image for the section of vessel of interest, or the amount ofvessel overlap in the projection image, could also be taken intoconsideration.

In an embodiment, the quality score may be visualized together with theangiographic X-ray image, the calculated functional index and,optionally, the determined segmentation of the vessel segment ofinterest.

In a further embodiment, if the functional index is independentlyderived for a plurality of images, the quality score may be calculatedfor each image together with the functional index. For example, thequality score may then be used as a weighting factor in determining acombined functional index, whereby a functional index derived from alower quality image is given less weight in the combined index as afunctional index derived from a higher quality image.

The approach described herein is applicable for functional assessment ofstenosis in all major arteries of the human body (coronaries, iliac,femoral, brachial, hepatic, carotids).

In an embodiment, the functional index may be compared to one or moregeometric measures for example obtained by means of QCA. Automatic QCAmeasurements may be carried out along the centerline of the vessel orvessel section of interest. Likewise, an FFR value may be calculated forthe same vessel or vessel section.

For example, a user interface may be provided that enables suchcomparison. Both the QCA and FFR values can be normalized and overlaidon top of the angiographic X-ray image that was used as a basis for thecalculations. In an example, portions of the vessel where a discrepancybetween the normalized QCA and FFR values exceeds a predeterminedthreshold may be determined and visualized on the image, for instance bymeans of highlighting or color coding. The normalization may be based onstandard decision thresholds, e.g. a threshold for a decision whether astent is to be placed or not. In that case, a QCA value of 0.5 maycorrespond to an FFR value of 0.8.

It has to be noted that embodiments of the invention are described withreference to different subject matters. In particular, some embodimentsare described with reference to a method whereas other embodiments aredescribed with reference to the apparatus. However, a person skilled inthe art will gather from the above that, unless otherwise notified, inaddition to any combination of features belonging to one subject matteralso any combination between features relating to different subjectmatters is considered to be disclosed with this application. However,all features can be combined providing synergetic effects that are morethan the simple summation of the features. While the invention has beenillustrated and described in detail in the drawings and foregoingdescription, such illustration and description are to be consideredillustrative or exemplary and not restrictive. The invention is notlimited to the disclosed embodiments. Other variations to the disclosedembodiments can be understood and effected by those skilled in the artin practicing a claimed invention, from a study of the drawings, thedisclosure, and the dependent claims.

In the claims, the word “comprising” does not exclude other elements orsteps, and the indefinite article “a” or “an” does not exclude aplurality. A single parameter, feature or other element may fulfil thefunctions of several items re-cited in the claims. The mere fact thatcertain measures are re-cited in mutually different dependent claimsdoes not indicate that a combination of these measures cannot be used toadvantage. Any reference signs in the claims should not be construed aslimiting the scope.

The invention claimed is:
 1. An apparatus, comprising: an inputinterface; and a processing unit, wherein the input interface isconfigured to obtain image data of at least a section of a vessel,wherein the image data comprises a single two-dimensional (2D)representation of at least the section of the vessel, and wherein theprocessing unit is configured to generate, based on the single 2Drepresentation, a functional index for stenosis assessment correspondingto a fluid flow through at least the section of the vessel, wherein, togenerate the functional index for stenosis assessment, the processingunit is configured to: determine a geometric model of at least thesection of the vessel based on the image data, wherein the geometricmodel comprises an inner diameter and a change of the inner diameteralong a course of at least the section of the vessel, wherein thefunctional index for stenosis assessment is distinct from the innerdiameter and the change of the inner diameter; determine a resistancevalue of at least the section of the vessel based on the geometricmodel; and determine the functional index for stenosis assessment basedon: the resistance value; and a boundary condition comprising at leastone of: a volumetric flow rate associated with an inlet of at least thesection of the vessel; the volumetric flow rate associated with anoutlet of at least the section of the vessel; a pressure associated withthe inlet; or the pressure associated with the outlet.
 2. The apparatusof claim 1, wherein the processing unit is configured to: apply adensitometry method to the image data to compensate for foreshorteningeffects in the image data when determining a width of the vessel.
 3. Theapparatus of claim 1, wherein the processing unit is configured to:apply a scaling factor to a width of the vessel in the image data; anddetermine the functional index for stenosis assessment based on thewidth multiplied by the scaling factor.
 4. The apparatus of claim 3,wherein the processing unit is configured to: receive a distance of thevessel from an image plane of the image data; and determine the scalingfactor based on the distance.
 5. The apparatus of claim 3, wherein theprocessing unit is configured to: segment the vessel along a course ofthe vessel such that there are multiple vessel segments; and apply aspecific scaling factor to the width of each one of the multiple vesselsegments.
 6. The apparatus of claim 5, wherein the processing unit isconfigured to segment the vessel based on a distance between a vesselsegment and a projection surface.
 7. The apparatus of claim 3, whereinthe processing unit is configured to: detect a reference element withinthe image data; and determine the scaling factor based on a known sizeof the reference element and the size of the reference element in theimage data.
 8. The apparatus of claim 1, wherein the processing unit isconfigured to: independently derive a respective functional index forstenosis assessment from a corresponding one of a plurality of 2Drepresentations.
 9. The apparatus of claim 8, wherein the processingunit is configured to: determine a variation between the respectivefunctional indices for stenosis assessment from different ones of theplurality of 2D representations; compare the variation to apredetermined maximal variation; and generate an acceptance criterionbased on the comparison.
 10. The apparatus of claim 1, wherein theprocessing unit is configured to calculate a quality score for thesingle 2D representation.
 11. The apparatus of claim 1, wherein theprocessing unit is configured to determine the functional index forstenosis assessment using a lumped element fluid dynamics modelincluding the resistance value and the boundary condition.
 12. Theapparatus of claim 1, wherein the single 2D representation comprises aprojection image.